1. Field of the Invention
The disclosed embodiments of the present invention relate to providing a reference voltage, and more particularly, to a bandgap reference circuit capable of providing a reference voltage having a voltage level below, for example, 1.25V.
2. Description of the Prior Art
A voltage reference generator is an essential design block required in analog and mixed circuits, such as data converters, phase lock-loops (PLL), oscillators, power management circuits, dynamic random access memory (DRAM) and flash memories. A voltage reference generator typically employs a bandgap reference circuit to generate a bandgap reference that is relatively insensitive to temperature, power supply and load variations.
Please refer to FIG. 1, which is a schematic diagram of an exemplary example of a conventional bandgap reference circuit 100. The conventional bandgap reference circuit 100 includes a transistor 110, a resistor 120 and a diode 130. The transistor 110 has a first connection node N1, a second connection node N2 and a control node NC. The resistor 120 has a first end E1 and a second end E2. The diode 130 has an anode and a cathode. The first connection node N1 of the transistor 110 is coupled to a supply voltage VDD, the second connection node N2 of the transistor 110 is coupled to the first end E1 of the resistor 120, and the control node NC of the transistor 110 is coupled to a bias voltage VBS. The second end E2 of the resistor 120 is coupled to the anode of the diode 130. The cathode of the diode 130 is coupled to an electrical ground GND.
The bias voltage VBS controls the transistor 110 to be enabled, thereby generating a proportional-to-absolute-temperature current IPTAT. If the value of the resistor 120 is R0, a cross voltage IPTAT×R will be yielded when the current IPTAT passes through the resistor 120. In this way, an output voltage Vout of the bandgap reference circuit 100 may be expressed as follows: Vout=VBE+IPTAT×R0, wherein the voltage VBE is the forward bias voltage of the diode 130.
Since the voltage VBE is the forward bias voltage of the diode 130, the voltage VBE has a negative temperature coefficient. That is, the voltage VBE decreases in response to temperature increase, or vice versa. Similarly, the cross voltage IPTAT×R has a positive temperature coefficient due to the electrical characteristics of both the transistor 110 and the resistor 120. As a result, the output voltage Vout of the bandgap reference circuit 100 may be immune to temperature variations when the voltage VBE complements the cross voltage.
The reference voltage outputted from a conventional bandgap reference circuit is usually about 1.25V, however, which is roughly equal to silicon bandgap energy measured at 0K in electron volts, whereas recent IC design typically requires operation regions below 1.25V. Thus, there is a need for an innovative bandgap reference circuit capable of providing a lower reference voltage.